Documentation

The parser p ? msg behaves as parser p, but whenever the
parser p fails without consuming any input, it replaces expect
error messages with the expect error message msg.

This is normally used at the end of a set alternatives where we want
to return an error message in terms of a higher level construct
rather than returning all possible characters. For example, if the
expr parser from the try example would fail, the error
message is: '...: expecting expression'. Without the (<?>)
combinator, the message would be like '...: expecting "let" or
letter', which is less friendly.

This combinator implements choice. The parser p <|> q first
applies p. If it succeeds, the value of p is returned. If p
fails without consuming any input, parser q is tried. This
combinator is defined equal to the mplus member of the MonadPlus
class and the (Control.Applicative.<|>) member of Control.Applicative.Alternative.

The parser is called predictive since q is only tried when
parser p didn't consume any input (i.e.. the look ahead is 1).
This non-backtracking behaviour allows for both an efficient
implementation of the parser combinators and the generation of good
error messages.

parse p filePath input runs a parser p over Identity without user
state. The filePath is only used in error messages and may be the
empty string. Returns either a ParseError (Left)
or a value of type a (Right).

The parser token showTok posFromTok testTok accepts a token t
with result x when the function testTok t returns Just x. The
source position of the t should be returned by posFromTok t and
the token can be shown using showTok t.

This combinator is expressed in terms of tokenPrim.
It is used to accept user defined token streams. For example,
suppose that we have a stream of basic tokens tupled with source
positions. We can than define a parser that accepts single tokens as:

The parser token showTok nextPos testTok accepts a token t
with result x when the function testTok t returns Just x. The
token can be shown using showTok t. The position of the next
token should be returned when nextPos is called with the current
source position pos, the current token t and the rest of the
tokens toks, nextPos pos t toks.

This is the most primitive combinator for accepting tokens. For
example, the Text.Parsec.Char.char parser could be implemented as:

The parser unexpected msg always fails with an unexpected error
message msg without consuming any input.

The parsers fail, (<?>) and unexpected are the three parsers
used to generate error messages. Of these, only (<?>) is commonly
used. For an example of the use of unexpected, see the definition
of Text.Parsec.Combinator.notFollowedBy.